The present disclosure relates to a method of depositing a kesterite film. More particularly, the present disclosure relates to a method of depositing a kesterite film from a precursor solution.
Large-scale production of photovoltaic devices requires high-throughput technologies and abundant environmentally friendly materials. Thin-film chalcogenide-based solar cells provide a promising pathway to cost parity between photovoltaic and conventional energy sources.
Currently, only Cu(In,Ga)(S,Se)2 and CdTe technologies have reached commercial production and offer over 10 percent power conversion efficiency. These technologies generally employ (i) indium and tellurium, which are relatively rare elements in the earth's crust, or (ii) cadmium, which is a highly toxic heavy metal.
Copper-zinc-tin-chalcogenide kesterites have been investigated as potential alternatives because they are based on readily available and lower cost elements. However, photovoltaic cells with kesterites, even when produced using high cost vacuum-based methods, at best only <6.7 percent efficiencies, see Katagiri, H. et al. Development of CZTS-based thin film solar cells; Thin Solid Films 517, 2455-2460 (2009).
The commonly owned applications: U.S. Pub. App. No. 2011/0094557A1, and PCT App. No. WO 2011/051012 to Todorov et al. and a publication by T. Todorov, K. Reuter, D. B. Mitzi, Advanced Materials, (2010) Vol. 22, pages 1-4, generally describe a hydrazine-based deposition approach of depositing homogeneous chalcogenide layers from mixed slurries containing both dissolved and solid metal chalcogenide species dispersions of metal chalcogenides in systems that do not require organic binders. Upon anneal the particle-based precursors readily react with the solution component and form large-grained films with good electrical characteristics. Recently, this process achieved world-record efficiency for this class of materials of 11.1% (T. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D. B. Mitzi, “Beyond 11% Efficiency: Characteristics of State-of-the-Art Cu2ZnSn(S,Se)4 Solar Cells”, Advanced Energy Materials, early view: DOI: 10.1002/aenm.201200348).
A major challenge in hydrazine-based copper-zinc-tin-chalcogenide kesterite processing including copper-zinc-tin-sulfide (CZTS), copper-zinc-tin-selenide (CZTSe), and copper-tin-zinc-sulfur-selenium (CZTSSe), is the poor solubility of the zinc chalcogenide-hydrazinates that generally form a solid phase in the ink. Unlike the various soluble chalcogenides compounds, zinc compounds such as ZnS and ZnSe, together with most transition metals and metal chalcogenides, show negligible solubility in hydrazine-based solvent systems. The morphology and dispersibility of the solid phase of these zinc compounds are difficult to control resulting in poor reproducibility of the hydrazine-based copper-zinc-tin-chalcogenide kesterite slurries that may cause micro-scale compositional non-uniformities, thereby potentially deteriorating device performance. Furthermore, particle-based inks may have poor compatibility with liquid-coating equipment such as slit-casting and spin coating due to non-Newtonian liquid properties of these slurries.
A pure solution precursor ink formulation for copper-zinc-tin-chalcogenide kesterite based on DMSO solutions was previously reported (W. Ki, H. Hillhouse Adv. Energy Mater. 2011, 1, 732-735). However, maximum efficiency reached only 4.1% possibly due to difficult to eliminate impurities introduced with the selected precursors. Another example employing sol-gel solutions in methoxyethanol reports 2.2% efficiency.